A new species of the genus Calliaxina Ngoc-Ho, 2003 from the South China Sea (Crustacea, Decapoda, Axiidea, Callianassidae)

Abstract A new species of the genus Calliaxina Ngoc-Ho, 2003, Calliaxina xishaensis sp. n., collected from the South China Sea is described and illustrated. It is distinguishable from Calliaxina thomassini Ngoc-Ho, 2014 by having the rostrum broadly triangular with pointed tip and is distinguishable from Calliaxina novaebritanniae (Borradaile, 1900) and Calliax punica (de Saint Laurent & Manning, 1982) by the posterior margin of telson being convex. It is also the first record of this genus from the China seas. A key to the species of Calliaxina is given.


Introduction
While working on the taxonomic study of the axiidean fauna (Crustacea, Decapoda) of the China Sea, an undescribed species assignable to the genus Calliaxina Ngoc-Ho, 2003 was found from Xisha islands, South China Sea. Ngoc-Ho (2003) established the genus Calliaxina mainly differing from Calliax de Saint Laurent, 1973 andEucalliax Manning &Felder, 1991 by having an exopod on maxilliped 3. Sakai (2005) considered Calliaxina as a synonym of Calliax, but later Sakai (2011) recognised Calliaxina as valid: he argued that the presence of a sulcus was considered significant in the classification of the genera, and the presence of an exopod on the maxilliped 3 was not "of vital importance", but he never-the-less expanded the genus to include eight species.
In this work, the classification of Calliaxina as defined by Ngoc-Ho (2003) is adopted, since the cardiac sulcus as an uncalcified suture running across the carapace between the cervical groove and the posterior border is regarded as significant, while the presence of an exopod on maxilliped 3 is a diagnostic character (Ngoc-Ho, 2014). Two species Eucalliax kensleyi Dworschak, 2005 and Callianassa bulimba Poore & Griffin, 1979 with rudimentary exopod on maxilliped 3 also should be belong to this genus.

Key to the species of the genus Calliaxina
Description. Carapace scattered with small shallow depressions, lacking the dorsal oval. The frontal margin bears a broadly triangular rostrum, acute terminally, not reaching middle of eyestalks in dorsal view (Fig. 1A). Lateral projections produced, nearly reaching tip of rostrum. Cervical groove distinct, conjunct with linea thalassinica. Distinct suture (linea anomurica) ventral to hepatic boss extending posteroventrally to ventral margin of carapace. Cardiac suture in middle posterior half of carapace well defined, incomplete across midline of carapace, extending anterioventrally to linea anomurica.
Antennular peduncle shorter but observably heavier than antennal peduncle (Fig.  1A); article 1 laterally and ventrally inflated; article 2 slightly longer than basal article, article 3 nearly 2/3 length of article 2; articles 2-3 with ventrolateral row of long, ventrally directed setae, continued onto ventral ramus of flagellum; rami of flagellum about equal length, nearly six times length of third article of peduncle; dorsal ramus with sparse short setae. Antennular peduncle (Fig. 1A) distinctly longer than antennal peduncle; article 1 with dorsolateral carina bearing regular line of fine setae above laterally produced excretory pore; article 3 shorter than article 2, with rudimentary scale on dorsal surface; article 4 elongate, longer than article 5 or combined length of first three.
Left and right pereopod 1 subequal, dissimilar in dentition of fixed fingers. Right cheliped (Fig. 1E) ischium slender, approximately 2.1 times as long as broad, upper margin almost straight, lower margin with 11-13 small denticles in middle. Merus about 1.8 times as long as broad, upper margin slightly convex, lower margin with 5-6 small denticles proximally. Carpus broad, increasing in breadth distally, lower margin arcuate, upper and lower margins keeled, terminating distally in triangular corners. Propodus heavy, 1.1 times as long as broad, inner surface of palm smooth; upper and lower propodal margins keeled, keel of lower becoming ill-defined beyond mid-length and absent on fixed finger, tufts of setae on inner face above lower margin; fixed finger thick, prehensile margin armed with one well-separated triangular tooth in mid-length, micro-serrations on upper margin of tooth and distally of it, distal 1/4 of finger unarmed, terminating in acute tip. Dactylus heavy, slightly longer than fixed finger, with pointed curved tip, unarmed.
Left cheliped (Fig. 1F) slightly smaller, similar in shape, cutting edge of fixed finger unarmed, but with a longitudinal triangular depression field of small tubercles on outer face; dactylus more slender than in major cheliped.
Pereopod 2 ( Fig. 2A) ischium 0.7 times as long as high; merus 2.4 times as long as high, upper margin smooth, lower margin protruding and with row of dense long setae; carpus subtriangular, shorter than merus; chela shorter than carpus, with dense setae on lower and upper margins; palm with upper margin slightly convex; dactylus 2.8 times as long as upper margin of palm; carpus and chela fringed with short to long setae along margins.
Pereopod 3 (Fig. 2B) simple, moderately slender. Ischium slender, approximately as long as high; merus approximately 2.0 times as long as high, upper and lower mar-gin slightly convex; carpus subtriangular, shorter than merus, broadest subdistally, approximately 1.4 times as long as high; propodus subrectangular, broadly flared distally to produce strong lower lobe, upper margin slightly convex and 0.7 length of carpus, with numerous tufts of setae on lateral surface and row of thick setae along upper and lower margins; dactylus subtriangular, upper and lower margins convex, outer surface densely setose, terminating in corneous tip.
Pereopod 5 (Fig. 2D) minutely chelate, all articles unarmed. Ischium rectangular; merus nearly 4.3 times as long as ischium; carpus approximately 0.8 length of merus, upper margin swollen; propodus 0.6 length of carpus, lower distal corner projecting to form a chela with dactylus, lateral surface beset distally with dense setae; dactylus hooked excavate, spooned, toward external side of fixed finger, tips of dactylus and fixed finger obtuse.
Telson (Fig. 1G) c. 1.9 times as broad as long, broadest at midlength, posterolateral margin rounded, with one tuft of setae each near lateral margin, posterior margin convex; dorsal surface with row of long setae at anterior 1/4 and a transparent, banded punctae near anterior margin.
Uropodal endopod (Fig. 1G) subovate, longer than telson, 1.6 times as long as wide; margins unarmed; with distinct submedian carina on dorsal surface. Uropodal exopod (Fig. 1G) broad, fan-shaped, almost as long as wide, posterodistal margin with thick spiniform setae and dense fringe of setae; with distinct submedian carina and dorsal plate on dorsal surface, distal edge of carina lined with short spiniform setae.
Variation. Maxilliped 3 exopod rudimentary in small specimen (paratype). Etymology. The species name is based on the type locality, Xisha islands, South China Sea.
Distribution and habitat. Presently only known from the type locality.
Calliaxina xishaensis sp. n. is the seventh species assigned to the genus. It is remarkably distinguished from the other species of the genus in the sixth pleomere somite with two lateral transparent, subrectangular punctae, and the dorsal surface of the telson with transparent, banded punctae.
The new species is closely related to C. novaebritanniae and C. punica in having the rostrum broadly triangular with a pointed tip, whereas C. thomassini has a minute or nearly absent rostrum. It can also be distinguished from C. novaebritanniae and C. punica by the convex posterior margin of the telson (versus straight). It is also similar to C. bulimba in the fixed finger of left cheliped bearing a longitudinal triangular depression field of small tubercles on its outer face, but It can be distinguished from latter by the distinct exopod on maxilliped 3 (versus rudimentary).